There have been various proposals for a method to guarantee the performance of a storage device or the like. These proposed methods use Input and Output per Second (IOPS) or throughput.
Incidentally, the busy rate of the storage device is an indicator that expresses how busy the storage device is, and for example, is calculated from the percentage of time that an IO request resides in an IO queue. Therefore, when the busy rate of the storage device is 100% (“1” in the case of using numerical values from 0 to 1), it means that it is not possible to obtain anymore performance. More specifically, when the busy rate of the storage device becomes 100%, it represents that IO requests always reside in the IO queue, and any IO is being executed, so the storage device is in a state where it would be difficult to increase the IO performance more than the present performance. IOPS, for example, is calculated from the number of accesses (the number of IOs) per unit time, which is counted in unit blocks.
Here, in case of accessing the storage device, even when trying to ensure the same IOPS, the busy rate of the storage device fluctuates and does not become the same value, when the state of the position and range on a disk of the storage device in which an IO occurs, the ratio of read and write instructions, and the distribution of the IO size change to a different state. For example, in case of considering the difference in range on the storage device in which an IO occurs, when comparing the case in which the occurrence of the IO is dominant in all of the storage areas, with the case in which the occurrence of the IO in part of the storage area is dominant, the overhead time other than for data transfer (for example, seek time) becomes greater in the former case, so even for the same IOPS, there is a tendency for the busy rate in the former case to become higher. In other words, this means that, depending on the occurrence status of the IO, it may be or may not be possible to guarantee the IO bandwidth using an indicator value such as IOPS or throughput.
In other words, there are no conventional arts to guarantee the bandwidth of the IO regardless of the occurrence status of the IO.